Recently, Microsoft released Internet Explorer 8, which boasted much better standards compliance than previous iterations of the browser. While it passed the Acid2 test, IE8 failed miserably in the Acid3 test, and many people criticised Microsoft for it. Microsoft Australia's Nick Hodge has stated that Microsoft purposefully decided not to support Acid3, because the test tests against draft standards.

The WEBKIT team know that CSS 3.1 isn't final; so, they prefix "WEBKIT-" before all non-standard and non-final standard tags. This is the recommended method of adding non-standard features to your browser.

Why can't Microsoft support the current standards in full CSS 2.1, and SVG 1.1. They could then add the "IE-" prefix to the CSS3.1 elements and SVG 1.2 elements. It is always assumed that the behavior of "Browser_Tagged" elements are subject to change in the future and you should use at your own risk. When the standard becomes finial the developers can verify that they implemented the element correctly and then remove the prefix tags.